ComEx - This library is being developed by PNNL to replace ARMCI. It supports the fewer platforms than ARMCI but may be more robust on modern ones like InfiniBand. See http://hpc.pnl.gov/globalarrays/support.shtml for details.

ARMCI-MPI - This library is a completely separate implementation of the ARMCI interface by Argonne and Intel. It uses the one-sided communication features of MPI, rather than a platform-specific conduit. It currents provides both MPI-2 and MPI-3 implementations; the MPI-3 one is supported by nearly all platforms. See https://wiki.mpich.org/armci-mpi/index.php/Main_Page for details.

By default, Global Arrays will choose ARMCI or ComEx, based upon the environment variables selected by the user (see GA documentation for details). When a native implementation is available and works reliably, this is the best option for the NWChem user. However, in cases where an native implementation of ARMCI is not available or is not reliable, the user should consider using one of the MPI-based implementations.

There are many different ways to use MPI as the communication runtime of Global Arrays:

It is difficult to provide complete guidance to the user as to which option to choose. However, we observe the following:

ARMCI_NETWORK=MPI-PR is stable and performs well on many platforms (including Cray XC platforms, e.g. NERSC Edison). This port will use one processes on each node for communication, therefore subtracting one process (again on each node) for NWChem. Therefore, when executing on a single node (i.e. the case of desktop execution) you would need to ask for n+1 processes; in other words, a serial execution would require the following mpirun invocation mpirun -np 2 ...

On Intel True Scale and Omni Path systems, MPI-PR is more reliable than OPENIB or MPI-SPAWN. ARMCI-MPI with Casper and Intel MPI is also recommended. See this page for details. Contact Jeff Hammond for assistance.

On IBM Blue Gene/Q, one must use the mpi2rma branch of ARMCI-MPI because MPI-3 is not fully supported. On all other platforms, the ARMCI-MPI branch mpi3rma is recommended.

The performance of ARMCI-MPI is greatly enhanced by Casper. See this link for design details and this page for instructions on how to use it.

When using ARMCI-MPI, please make sure to use the most recent version of MPI (MPICH 3.2+, Cray MPI 7.2+, MVAPICH2 2.0+, Intel MPI 5.1+, Open-MPI 2.0+). Older versions of MPI are known to have bugs in the MPI-3 RMA features that affect the correctness of NWChem.